To English speakers, the distinctions between blue and green, cup and glass, or cut and break seem self-evident. The intuition is that these words label categories that have an existence independent of language, and language merely captures the pre-existing categories. But cross-linguistic work shows that the named distinctions are not nearly as self-evident as they may feel. There is diversity in how languages divide up domains including color, number, plants and animals, drinking vessels and household containers, body parts, spatial relations, locomotion, acts of cutting and breaking, acts of carrying and holding, and more. Still, studies documenting variability across languages also uncover striking commonalities. Such commonalities indicate that there are sources of constraint on the variation. Both the commonalities and divergences carry important lessons for Cognitive Science. They speak to the causal relations among language, thought, and culture; the possibility of cross-culturally shared aspects of perception and cognition; the methods needed for studying general-purpose, nonlinguistic concepts; and how languages are learned